Conventionally, the use of user interface devices is widespread, and the user interface devices enable a user's input motion or operation by manipulating a display screen with the user's finger, stylus, etc. Notably, in recent years, an increasing number of mobile information devices typified by such as smart phones, tablet computers, electronic books or PDAs (Personal Digital Assistants) have incorporated touch panels serving as a user interface device that receives a finger's motion.
Irrespective of whether a touch panel is incorporated or not, a mobile information device is demanded to be downsized for a greater mobility. When a mobile information device is equipped with a user interface device in the form of a plurality of physical input keys or buttons, these keys themselves are needed to be reduced in size, resulting in a risk that the operability of the mobile information device becomes poorer. In contrast, when a mobile information device is equipped with a user interface device in the form of a touch panel using a plurality of virtual buttons instead of physical buttons, these virtual buttons can be well adjusted in size depending on functions assigned to these buttons, resulting in easy designing process for improving the operability of the mobile information device.
On the other hand, the use of a touch panel is more likely to induce incorrect input actions than when physical keys are used instead. More particularly, even a user's unintended touch with a touch panel can be recognized or interpreted as an input action, or a user's intended correct input action can be misrecognized or misinterpreted, failing to activate a desired function, and requiring the user to repeat the same input action.
For dealing with the problems with incorrect input, for example, Patent Document No. 1 discloses a technique of varying the length of a cycle time for repeatedly detecting a user's touches on a touch panel after the user's initial touch on the touch panel is detected. This technique can distinguishably or distinctly recognize the user's brief touch occurring on the touch panel for a very short time, and the user's scrolling action by sliding the user's finger on the touch panel.
In addition, Patent Document No. 2 discloses a technique of displaying a plurality of user-selectable buttons on a touch panel, such that at least one of the displayed buttons is enlarged and rearranged, in response to the touch panel being touched by a user's finger, to thereby help the user to select a desirable one of the buttons. Further, Patent Document No. 3 discloses a technique of vibrating a multi-touch sensitive touch panel enabling detection of concurrent multiple touches on the touch panel, to thereby help a user in tactually perceiving that the user has performed a particular input motion.
It is noted that there is also a technique of detecting a user's press occurring on a touch panel having an electrically conductive film interposed between two opposed elements, and of measuring the intensity of the press (See Patent Document No. 4). Further, there is still also a technique of allowing a user to visually perceive how strong the user has pressed paired touch panels during the user's input motion by touching the two touch panels, based on the user's depth sensation perceived from images displayed on the paired touch panels (See Patent Document No. 5).